Naproxen has been administered for 22 febrile episodes of unknown etiology in 21 patients with malignant tumors. Ten patients had malignant lymphomas, and the others had solid tumors. The highest temperature was 38.5 degrees C or more in all but one of the patients. Out of the 22 episodes, 2 episodes were found to have been caused by an infection. The naproxen doses ranged from 300 to 600 mg per day. Of the 20 episodes of neoplastic fever, the temperature decreased 1.0 degree C or more in 16 patients, in 13 of whom the temperature decreased to 37.0 degrees C or less. There was some evidence that suggests the existence of a dose-response relation-ship. The initial use of 600 mg per day seemed to be appropriate. The antipyretic effect did not seem to be influenced by the type of the underlying tumors. Three patients continued to receive naproxen for one to to months. No side effect was observed, excepting for one patient who developed a mild nausea. We thus conclude that naproxen is an effective safe drug for the control of neoplastic fever.